Plasma devices are frequently used in manufacturing semiconductor devices and flat-panel displays, in forming oxide films and in such processes as crystal growth, etching and ashing, of semiconductor layers. Some plasma devices, referred to as high-frequency plasma devices, use a slot antenna for supplying a high-frequency electromagnetic field into a process vessel to generate a high-density plasma by means of the electromagnetic field. One feature of the high-frequency plasma device is that this device is fit for a wide range of uses because of its ability to generate a plasma stably even if a plasma gas has a relatively low pressure.
FIG. 11 shows an exemplary structure of a conventional high-frequency plasma device. The structure in FIG. 11 is partially shown in vertical cross section.
The plasma device has a process vessel 111 in the shape of a bottomed cylinder with its top opened. On the bottom of process vessel 111, a substrate platform 122 is fixed. This substrate platform 122 has a mount surface on which a substrate 121 to be processed is mounted. A nozzle 117 for supplying a plasma gas is provided in the sidewall of process vessel 111, and an exhaust vent 116 for vacuum pumping is provided in the bottom of process vessel 111. The top opening of process vessel 111 is closed with a dielectric plate 113 for preventing the plasma from escaping to the outside.
A radial antenna 130 which is one type of the slot antennas is provided above dielectric plate 113. This radial antenna 130 is constituted of two circular electrically-conductive plates 131 and 132 parallel to each other that form a radial waveguide 133, and an electrically conductive ring 134 which connects respective circumferential parts of conductive plates 131 and 132.
Conductive plate 131 forming the lower surface of radial waveguide 133 has a plurality of slots 136 formed therein. As shown in FIG. 12, slots 136 are concentrically formed in the circumferential direction which is orthogonal to the radial direction of conductive plate 131.
At the center of conductive plate 132 forming the upper surface of radial waveguide 133, an inlet 135 for electromagnetic field F is formed, and a high-frequency generator 144 is connected by a cylindrical waveguide 141 to inlet 135. In addition, a circular polarization converter 142 is provided at cylindrical waveguide 141 for feeding radial antenna 130 with circularly polarized electromagnetic field.
Further, the circumferential parts respectively of radial antenna 130 and dielectric plate 113 are covered with an annular shield member 112 to form a structure preventing leakage of electromagnetic field F to the outside.
Electromagnetic field F supplied from high-frequency generator 144 is converted into circularly polarized electromagnetic field by circular polarization converter 142 to be fed to radial antenna 130. Electromagnetic field F supplied into radial waveguide 133 is radially propagated from the center toward the periphery of radial waveguide 133. Electromagnetic field F then reaches conductive ring 134 and is reflected therefrom toward the center again. In this way, electromagnetic field F is propagated in radial waveguide 133 while radiated little by little from a plurality of slots 136. Electromagnetic field F radiated from slots 136 passes through dielectric plate 113 to ionize the gas in process vessel 111 and accordingly generate a plasma in an upper space S above substrate 121.
FIG. 13 conceptually shows wavefronts of electromagnetic field F at a certain instant in radial waveguide 133. A progressive wave of electromagnetic field F has the helical wavefront as indicated by the solid line while a reflected wave has the helical wavefront as indicated by the dotted line. Magnetic flow Im1 of the progressive wave and magnetic flow Im2 of the reflected wave are generated along respective wavefronts.
As discussed above, conventional radial antenna 130 has slots 136 formed in the circumferential direction of conductive plate 131 serving as a radiation surface. The angle of inclination of magnetic flow Im2 of the reflected wave relative to slots 136 is almost equal to the angle of inclination of magnetic flow Im1 of the progressive wave relative thereto, therefore, not only the progressive wave but also the reflected wave tends to be radiated from slots 136.
In some cases, however, it is undesirable that both of the progressive wave component and the reflected wave component are included in electromagnetic field F radiated from slots 136. For example, when the intervals between the slot are used for adjusting the direction of radiation of electromagnetic field F, standing waves are radiated if the reflected wave is included at a higher ratio. Then, electromagnetic field F cannot be radiated in a desired direction.
Even if the standing wave appearing in radial waveguide 133 is to be used for any positive purpose, the radiation efficiency is unsatisfactory and thus efficient supply of electromagnetic field F into process vessel 111 is impossible.
The present invention has been made to solve the above-discussed problems. One object of the present invention is to selectively take a desired component of the electromagnetic field in the slot antenna for supplying the selected component into the vessel for use in plasma generation.
Another object of the present invention is to efficiently supply the electromagnetic field in the slot antenna into the vessel for use in plasma generation.